The present invention generally relates to carbonic acid gas sensors and, more particularly, is directed to a sensor used for measuring a carbonic acid gas dissolved in a body fluid or a fluid such as water in a river.
A Severinghaus-type carbonic acid gas sensor has been most popularly employed as a carbonic acid gas sensor for measuring a carbonic acid gas dissolved in a fluid. The Severinghaus-type carbonic acid gas sensor is described, for example, in "Theory of Measurement of a Blood Gas and Clinical Application thereof" by Takanori Fujiwara, Shinkou Koueki Medical Book Publishing Section, pp. 150 to 152.
In the Severinghaus-type carbonic acid gas sensor, a gas permeable membrane of tetrafluoroethylene, for example, is attached at a tip portion of a sensor body, and an electrolyte solution including sodium bicarbonate is filled in the sensor body. Further, the sensor employs a pH-electrode having an internal electrolyte solution and employs a glass membrane as a pH sensitive membrane for the pH-electrode. The pH-electrode is mounted in the sensor so as to closely contact with the gas permeable membrane through the electrolyte solution. In the thus-constituted sensor, when the sensor contacts with a solution to be measured, a carbonic acid gas is diffused into the electrolyte solution including sodium bicarbonate through the gas permeable membrane and dissolves in the solution, so that the pH of the electrolyte solution changes. This change of pH can be detected by the pH-electrode provided in the sensor. According to this theory, the partial pressure of the carbonic acid gas in the solution can be measured.
A carbonic acid gas sensor which is fabricated by miniaturizing the Severinghaus-type carbonic acid gas sensor on the basis of semiconductor manufacturing techniques is described in "Proceeding of the Symposium on Biosensors" 1984, pp. 33 to 34. This carbonic acid gas sensor employs, instead of the pH-electrode of the above-described sensor, an ion-sensitive field effect transistor (ISFET) which is an all solid type sensor having no internal electrolyte solution. In this carbonic acid gas sensor, the ISFET is disposed in a catheter tube, an electrolyte gel is filled on a pH sensitive membrane which is a gate of the ISFET and then the gel is covered by a gas permeable membrane.
Another example of a carbonic acid gas sensor, in which a gas permeable membrane and an electrolyte gel as well as a pH-electrode are embedded by utilizing semi-conductor manufacturing techniques is described in "Sensors and Actuators B2", 1990, pp. 291 to 295. In this carbonic acid gas sensor, a pressure membrane of polyimide is coated on a sensor substrate, and a groove is formed on a gate of an ISFET. Then, an electrolyte gel is filled in the groove, thereby forming a gas permeable membrane.
The firstly-described conventional Severinghaus-type carbonic acid gas sensor is a sensor of a type assembled on the basis of mechanical structure, and so has a life time of more than one year as long as it is not used in an inferior environment, and therefore has a high reliability. However, since each of the parts constituting the Severinghaus-type sensor is fabricated by mechanical processes, there have been problems difficult to be overcome in automatization of assembling processes and improvement of mass productivity that have been difficult to overcome.
In the secondly-described conventional carbonic acid gas sensor fabricated on the basis of the semiconductor manufacturing techniques, the ISFET serving as a pH-electrode has a high mass productivity. However, since all elements of the sensor including the pH-electrode, tube and gas permeable window or the like have not been embedded yet, there has been a problem in automization of assembling processes of the sensor and mass productivity. Further, since, after filling of the electrolyte gel on the pH sensitive membrane, the electrolyte gel is covered by the gas permeable membrane, there has been a problem that a distance between the gas permeable membrane and a pH sensitive portion of the ISFET is determined in accordance with an amount of the filled electrolyte. The response time of the sensor is influenced by this distance, so that the response time fluctuates greatly depending on the amount of the filled electrolyte.
In the thirdly-described conventional carbonic acid Gas sensor in which the Gas permeable membrane is embedded on a semiconductor substrate, an amount or capacity of the electrolyte gel is determined by the capacity of a groove formed on the gate of the ISFET. Since an area of the gate of the ISFET is small, the groove is required to be deeper in order to increase a capacity of the electrolyte gel. However, the deeper the groove is, the lower the response speed of the sensor becomes, so that the capacity of the electrolyte gel is limited. Thus, there has been a problem that the life time of the sensor is short.
As other related prior art, there have been known Japanese Patent Laid Open Publication Nos. (JP-As) 61-88138, 63-26569 and 63-279154.
JP-A 61-88138 discloses an electrochemical apparatus which has a laminated structure of a sheet-shaped solid electrolyte and a sheet-shaped electrode disposed in contact with the solid electrolyte.
JP-A 63-26569 discloses an ion selective electrode apparatus in which a path for the medium to be measured is provided, an electrolyte solution such as human blood is flown through the path, and then an ion concentration of the electrolyte solution is measured by utilizing a reference electrode and an indicator electrode disposed in a mutually insulated state.
JP-A 63-279154 discloses a sensor for detecting carbon dioxide which is constituted in a manner that recesses are formed on the entire surface of a semiconductor substrate, an agarose gel including an electrolyte solution is filled in the recesses, and then the recesses are covered by a gas permeable membrane.